1. Field of the Invention
The present invention relates to a distribution automation remote terminal unit (DARTU) for use in a distribution automation systems particularly for electrical power distribution.
2. Prior Art
Generally speaking, such electrical power distribution has geographically remote plant units or installations forming the distributed network. They usually have one or more RTUs which have some or all of the system functions or measurement, protection, control and communications and these RTUs include suitable memory and signal processing means for said functions. Transmission of electrical supplies by utility companies is becoming more complex and expensive.
Customer service requirements are such that there is now little tolerance of instances of loss-of-supply and commercial considerations dictate a reduction in the number of personnel required to operate the network both for normal day-to-day operation and emergency fault-finding and maintenance. Furthermore, environmental considerations require electrical utilities to be able to shed load from portions of the network as required in circumstances. This is particularly important in areas which have a steadily growing demand for electricity which cannot be supplied by environmentally friendly methods.
Monitoring and/or automation of each node of the distribution network is therefore essential to ensure continuous and most efficient use is made of the power supplied. For example, control of the primary distribution network is particularly critical in that while it represents a relatively small number of network nodes it has a very significant impact on customers in terms of fault recovery.
Automation of these sites has typically been performed by the extensive use of RTUs. Such devices accept facility status information, control items of plant and measure analogue quantities either by directly accepting transducer outputs or by direct connection to the analogue quantities involved. A wide variety of communication schemes are utilised to link these units to a central location from which the whole network may be monitored and controlled.
Large number of RTUs are installed across the distribution network to monitor the state of the network and respond depending on the state of the network by running autonomous networks control functions, reporting to a master control station or reporting to a central master station controlling the entire network.
Historically these RTU units have been used to perform supervisory control and data acquisition (SCADA) and separate units have been used to perform separate protection functions. Both of these functions are vital for the continued performance of the network. Protection functions which are generally considered to be the most important aspect of operations utilise devices such as relays to control a variety of operations such as breaker protection and cable protection. SCADA systems involve the collection of information and the control of various points in the network including transmission sub-stations, distribution sub-stations and distribution networks.
Presently available DA RTU devices are either transducer based RTUs or transducerless RTUs. Transducer systems are based on xe2x80x9cscaling downxe2x80x9d a conventional sub-station RTU and the use of external transducers for AC voltage and current. These external transducers seriously compromise the ability of the RTU to derive calculated quantities and to determine the presence and direction of fault current. Such devices are not suitable for use in many protection schemes. In transducerless DA RTUs the external transducers are eliminated as AC voltages and currents are connected directly to the RTU via suitable transformers. These signals are measured which allows the RTU to accurately determine the values of voltage and current, derive calculated quantities and determine the presence and direction of fault current.
U.S. Pat. No. 5,237,511 (Caird et al) describes one construction of RTU for overcoming the problems of previously known RTUs which was an improvement on the RTUs known before that date. However, while this RTU overcomes some of the problems inherent in the previously known RTUs, there is still the problem of shared memories and components which means in the event of one of the parts of the RTU falling, the whole functionality of the RTU is destroyed.
There is therefore a need for an improved DA RTU which will overcome these problems.
In this specification, the term xe2x80x9cfunctionxe2x80x9d has been used very broadly to cover whatever an RTU might be required to do, whether it be simply recording, measurement, storing, controlling, communicating and so on. There is no limit to the functions which may be performed and the term xe2x80x9cfunctionxe2x80x9d is used in that general sense.
The term xe2x80x9cfailurexe2x80x9d is used but xe2x80x9cfailurexe2x80x9d may not necessarily mean absolute failure of a particular RTU or indeed absolute failure of portion of the RTU but it is used in the sense of any malfunction that would lead to inappropriate operation or use of a DA RTU.
The term xe2x80x9chealth messagesxe2x80x9d is used in this specification as a shorthand way of describing the reporting of information from one unit to another unit, which information relays information to the other unit which allows the other unit to assess the operating efficiency and availability of the unit sending the message.
The terms DARTU and RTU are used generally and include any form of controller used for distribution systems and not just conventional ones.
According to the invention, there is provided a distribution automation remote terminal unit for a control system having a master control station communicating with plant units each having one or more RTUs having some or all of the system functions of measurement protection control and communications and including suitable memory and signal processing means for said functions characterised in that the distribution automation remote terminal unit comprises:
at least two separate independently operable units with separate power supplies and I/O ports; and
an inter-unit communications means and in which one of the units is assigned the function of communication with the master station.
By separating the communications function from the remainder of the functions and by having independent functions, the failure of one unit does not cause the failure of the other.
Ideally, diagnostic means are provided in each unit for fault analysis and reporting. It is very important that any failure be realised and if the inter-unit communication means reports failure and the failure is on separate units having a DA RTU with protection, communication and control capabilities, then it would be possible for the control and the protection facility to continue to be operated in spite of the communications unit malfunctioning. If it is a different function which cannot be performed than the fact can be reported.
Preferably, the inter unit communication means is arranged to constantly deliver health messages from respective diagnostic means between the units and in which the units assigned the function of communication delivers said health messages to the master station as appropriate. It is very important that the master station be warned of any malfunction of any of the units.
Preferably, there are two units, namely a measurement and protection unit and then a communications and control unit. There is a considerable advantage in keeping the protection function separate from the communication function. For example, if the communication function fails, then the protection function still operates, while if the protection unit fails, the communications unit can notify the master station and the control unit can then be operated via the communications unit to ensure that whatever should be done with the protection unit failing, will be done. This may include, for example, manual or remote operation.
In one embodiment of the invention, the communications and control unit has means to operate the installation in response to the failure of some portion of the measurement and protection unit. By having additional means on the control means, sufficient to operate the installation when the protection unit is malfunctioning, is particularly advantageous. It does not have to be the optimum way to operate the plant unit but simply an adequate way.
In one embodiment of the invention, the measurement and protection means includes enabling means so that when it receives indication of the failure of some portion of the communications and control means the measurement and protection means continues to operate. It is very important that the measurement and protection means continue to operate even if communication with the master station has been terminated. In this latter embodiment of the invention, the two units comprise two separate physical modules and ideally the modules communicate to a dual port memory mechanism. By having two separate modules, it is possible that when one only needs to use, for example, a measurement and protection function to simply use that module and not to provide a module for the function of communications and control. Similarly, in some cases, communications and control will be required and there will be no need for measurement and protection. It also facilitates the adding of the additional module and thus the function at a later date.
In one embodiment, one port is for transfer of control information from the communication and control unit to the measure and protection unit and the other to transfer control information in the opposite direction.
Preferably, the modules communicate through a high speed memory contents sharing mechanism.
Ideally, the measurement and protection unit is divided into at least two separate sub-units, namely a measurement sub-unit and a protection sub-unit, each with an independent power supply and I/O ports and connected together by a sub-unit communications means. The advantage of having separate sub-units is that you remove the possibility of one of the sub-units being inoperable due to the failure of the other. The less facilities are shared and the more independently operable the units are, the better. However, in normal operation the ability to share the information and controls at high speed is very important.
Similarly, the communications and control unit is divided into at least two separate sub-units, namely a communications sub-unit and a control sub-unit, each with an independent power supply and I/O ports connected together by a sub-unit communications means.
Ideally, an automation system for an electrical power distribution network should have DA RTUs of the type hereinbefore described.